If we consider both good and bad games, the most overrated game on the RPGCodex is Planescape: Torment. Completely linear, with horrible encounters, zero replayability, pretentious and edgy writing (which is still better than in ~99% of games), and a very weak last part (Curst especially). It's still one of my favorite games, but the older I get, the harder it is to deny it's overrated.
If we're only looking at bad games, then the most overrated game on the RPGCodex is Morrowind. I'm not going to say that it's outright a no-go game. But in my opinion, unlike PS:T, it's a really weak game that looks particularly bad as a sequel to Daggerfall. Everything has been simplified: factions, reputation, magic, skills, loot, banks, mounts, ships, and so on. "Hand-created" dungeons look even more samey than those randomly generated from the previous installment. I understand that the game is loved for the weird atmosphere and lore (also music). But, in my opinion, it doesn't pull the game out of the swamp of mediocrity.